The present invention relates to capillary priming of positive displacement pumps in an environment of zero-gravity.
With the advent of space travel, and the building of space stations capable of sustaining human life in zero-gravity conditions, as well as the possibility of actual planetary colonies being founded, many changes must be made in the way conventional mechanical systems are operated. Everyday tasks which are made easier by machines must now be redesigned to assure their operation in zero gravity or near zero-gravity environments.
One important life support function which must be supplied on space stations is heating and cooling. Generally, these heating and cooling systems use the specific heat and the latent heat condensation and the heats of vaporization of fluids to add and remove heat from various areas of the space station.
Fluid transportation is an essential requirement in such heating and cooling systems. Fluids from the condensation of vapors must be moved to areas where they may again gain heat by vaporization. The pumping of fluids in a zero-gravity environment raises special problems due to the lack of any driving force to cause the liquid to fill the pump. On earth, pumping chambers are filled due to the effect of gravity or pressure heads. No such driving force is available in zero-gravity environments.
In positive displacement pumps, a definite volume of liquid is trapped in a chamber, which is alternately filled from an inlet and emptied at higher pressure through a discharge outlet. One example of a type of positive displacement pump uses a reciprocating piston or plunger to supply the necessary displacing force. Liquid is drawn through an inlet port into a cylinder by the withdrawal of the piston and then forced out through a discharge check valve on the return stroke. It is possible to use a piston pump which is double acting, i.e., the liquid is admitted alternately on each side of the piston, so that one part of the cylinder is being filled while the other is being discharged. Further, more detailed discussion of such pumps and their design may be found in Unit Operations of Chemical Engineering, 3rd Edition, McCabe, W. L., and Smith J. C., McGraw Hill, 1976, pp. 180-194 and Chemical Engineers Handbook, 5th Edition, Perry, R. H. and Chilton, C. H., McGraw Hill, 1973, pp 6-3 to 6-15.
An important step in the operation of a pump involves the introduction of fluid into the pumping chamber prior to displacement (i.e., priming the pump). Under normal conditions found within the earth's atmosphere (i.e., gravitational force, atmospheric pressure and temperature), several driving forces cause the fluid to flow into the pumping chamber. For example, gravity may supply a sufficient driving force to cause the fluid to flow into the chamber. Some type of liquid pressure head on the fluid inlet of the pump, such as submerging the fluid inlet of the pump in a tank of fluid, may cause the pumping chamber to become filled with fluid. Also, the suction force caused by the piston being reciprocated back may be sufficient to draw fluid into the pumping chamber. All of these methods are clearly operable under normal gravitational conditions (i.e., gravitational force on the earth's surface). However, when such pumps are placed in zero-gravity or near zero-gravity conditions, such as on a space station or the moon, severe difficulties arise in attempting to prime the fluid into the pumping chambers.
First, the lack of any gravitational forces rules out the most common driving force for priming the pump. Second, it is difficult to create a pressure head of fluid because to do so it is necessary to have gravity acting on the fluid to create the pressure head. In zero-gravity space applications, it is very possible that many fluids which need to be pumped may be saturated. Therefore, using suction on the fluid suffers from the drawback that the reduced pressure may cause the fluid to vaporize, especially if the fluid is saturated.
Another method of zero-gravity pumping of liquids is to spin the fluid in a centrifuge-type pump, and drain the liquid which accumulates on the periphery of the rotating chamber. However, these pumps suffer from the drawback that they require high speed motion, develop very small pressure heads, and are bulky. Also, at zero-gravity, gases and vapors must be carefully removed from liquid systems in order to pump them reliably with existing pump designs. Since the pumping of liquids will be necessary for space station experiments, utilities, and propulsion equipment, the present design approaches, requiring purging all gases and vapors, will be very maintenance intensive.
It would be desirable to provide a pump for use in zero-gravity conditions which does not suffer from the drawbacks of present day pump designs.